Cache for RegExp to improve performance

[ikkisoft]

With the current implementation, each regexp lookup involves filesystem access

$ inotifywait -mr ./
./ OPEN serialkiller.conf
./ ACCESS serialkiller.conf
./ CLOSE_NOWRITE,CLOSE serialkiller.conf

To improve performances, we could create a cache for all blacklists/whitelists - dropped whenever configuration reloads. Potentially, we could also make that cache static and/or thread-local and dependent on the filename, so that two SerialKillers with the same config share the cache

[ikkisoft]

Implemented in v0.3

[ikkisoft]

I've reverted 9ef6920 as it didn't seem to work properly on the latest version of Commons Configuration. I need to rethink the strategy here.